1. Field Of The Invention
This invention relates to an apparatus for controlling the flow of liquid metal, and more particularly, to an on/off valve for use with an electromagnetic flow control device.
Electromagnetic flow control devices, or valves (EMVs), have been demonstrated for many industrial applications. For example, it has been found that EMVs are particularly well suited for controlling liquid metal flow in continuous casting lines. U.S. Pat. No. 4,842,170 illustrates how EMVs are typically used in such applications. EMVs may also be used in billet or slab casters. In these types of casters, EMVs may be used to regulate the flow of liquid metal through a nozzle from a tundish to a mold.
EMVs are not suitable for use as shut-off valves for stopping the flowing metal. The liquid metal flowing through the EMV removes excess heat generated by the EMV. Using the EMV to stop the flowing metal could cause the metal to overheat and atomize, which is undesirable in a casting operation.
In existing casting operations, several devices have been used to stop the flow of liquid metal through a nozzle. One device is a chill-plug. A chill-plug is typically a conical-shaped copper plug which is inserted into the lower, or discharge, end of the nozzle. The chill-plug freezes the metal in the nozzle, thereby forming a column of solid metal which stops the flow. Chill-plugs may be used in open pour continuous casting operations where the nozzle is relatively small. Chill-plugs have not been adaptable for use in slab casters. The nozzle of the slab caster often is too large for the chill-plug to freeze a sufficient amount of metal to adequately stop the flow through the nozzle.
In addition, in order for a chill-plug to be used, the discharge end of the nozzle must be accessible, as in open pour operations. Many continuous casting operations today use shrouded flow, whereby a shroud extends from the discharge end of the nozzle to the mold. The shroud forms a conduit for the flowing metal and keeps contaminants from ingesting into the stream of metal. The presence of the shroud makes the lower end of the nozzle inaccessible, thereby making the use of the chill-plug impractical.
After the flow of metal has been stopped, it is usually necessary to restart the flow after some elapsed time. In operations using chill plugs, restart of the stopped flow is accomplished using an oxygen lance. The flame of the oxygen lance is applied to the discharge end of the nozzle to melt the column of frozen metal, thereby allowing metal to begin flowing again. A disadvantage of using an oxygen lance is that it may damage the refractory material from which the nozzle is made. The damage to the nozzle may increase the size of the nozzle orifice. Any variation in the size of the nozzle orifice may change the flow rate of the liquid metal through the nozzle, thereby requiring adjustments to the overall caster flow rate. Such adjustments are typically made by changing the level of the liquid metal in the tundish or may be made using an EMV. In addition, the damage to the nozzle shortens the operational life of the nozzle, which may increase the down time required for maintenance and restart processes.
Another type of device used to stop the flow of liquid metal is a slide gate. A slide gate is mounted on the lower, or discharge, end of the nozzle such that an opening through the slide gate is in communication with the opening through the nozzle. A slide plate may be positioned over the opening through the slide gate to block the flow of metal therethrough. The slide plate may also have a plurality of openings therethrough which allow the metal to flow through the slide plate to maintain and regulate the flow of metal.
A slide gate used to block the flow of metal can only be used to stop metal flow for short periods of time. If freezing of the metal occurs in the slide gate plates, there is no way to restart the flow without disassembling the slide gate. In addition, a slide gate typically uses a cumbersome and complicated actuator mechanism to operate the slide plate. One reason such a mechanism is required is that the slide plate must often be kept in constant vibratory motion to keep the metal adjacent to the slide plate from freezing and thereby preventing actuation of the slide gate. Because of the cumbersome control mechanism, slide gates are relatively large devices which are not well suited for use in smaller nozzle continuous casting operations, particularly those operations which utilize shrouded flow.
Another type of device commonly utilized to stop the flow of liquid metal is a stopper rod. A stopper rod is inserted into the upper end of the nozzle. This type of device can be used with continuous casters which utilize shrouded flow because no access to the lower end of the nozzle is required. However, this type of device requires mechanical linkages above the tundish to position and activate the stopper rod. The linkage and stopper rod require maintenance and their life is relatively short. In addition, the use of a stopper rod may damage the refractory material at the entrance to the nozzle.
A need exists for a compact device that can be utilized with an EMV flow control device to stop and restart the flow of liquid metal through a nozzle without causing damage to the nozzle or requiring complex activator mechanisms to operate. The device should permit flow to be restarred after the metal in the nozzle has frozen. Operating the device should not require access to the lower or discharge end of the flow nozzle so that the device can be utilized with casters which use open pour or shrouded flow processes.